Silicon used for manufacturing solar cells is produced commonly by the so-called Siemens method. The Siemens method is well-established, and is commonly used to manufacture solar cells. In the Siemens process, silgrain (>99.6% purity) is used. Silgrain is then reacted with hydrochloric acid in the presence of a copper catalyst. The main product obtained is trichlorosilane (SiHCl3), which is purified through fractional distillation. The separated SiHCl3 is decomposed and reduced at high temperature on high-purity silicon rods. For each mole Si converted to polysilicon, 3 to 4 moles of SiCl4 is produced, binding large amounts of chlorine and valuable silicon. The resulting polysilicon has typically an impurity level of 10-9. The Siemens method produces high quality silicon. However, the Siemens method is not well suited to meet the dramatic increase in demand over the past few years for silicon at competitive prices. In addition, it involves dangerous raw materials such as HCl, SiHCl3, and H2 during the manufacturing process and produces a poisonous by-product, SiCl4. The Siemens method is dangerous, and it is hazardous to personnel and the environment.
Another purification method is disclosed in Japanese Patent No. 3205352 (JP352), the teachings of which are incorporated herein by reference. JP 352 method involves removing phosphorous by high temperature electron beams, removing impurity with directional cooling, adding water and gases to remove boron carbide during plasma irradiation, and removing impurity with directional cooling again. Each of these steps is performed sequentially. Although JP352 uses a simple structure to perform silicon purification, the simplicity of the design appears to result in various purification steps interfering with each other, which compromises the purification process.
A more cost-effective method for purifying metallurgical silicon is needed to meet the increasing demand for purified silicon.